Subatomic question - K alpha lines

[mathtrek]

Having some difficulty with a subatomic problem concerning K-alpha lines:

 

"Light from the K-alpha line of an unknown material X is compared with the K-alpha line of Carbon. The wavelength ratio is lambda(x)/lambda© = 0.148. What is the matieral X?"

 

I've got no idea where to go with this one. The only time i've used K-alpha lines is in regards to changes of energy levels. I know E=hf=hc/lamba, but i don't see how that helps

[mathtrek]

Update - ok! managed to work it out. Good old Mosely's law!